The present invention relates to III-nitride power semiconductor devices.
The large dielectric breakdown field (>2.2 MV/cm) and the high current density of the two dimensional electron gas (2-DEG) in III-nitride heterojunction semiconductor devices make them attractive for power applications.
One known III-nitride heterojunction power semiconductor device is a high electron mobility transistor (HEMT). A desirable variation of a HEMT is a normally-off HEMT; i.e. a HEMT which does not allow for conduction of current (except for minute leakage current) in the absence of an appropriate voltage to its gate electrode.
FIG. 1 of U.S. patent application Ser. No. 11/537,304 (assigned to the assignee of the present application) illustrates a normally-off III-nitride power semiconductor device, which includes a III-nitride semiconductor stack. The stack so disclosed includes an N-polar GaN layer, an N-polar AlGaN layer, an N-polar GaN layer, and an N-polar AlGaN layer disposed below the gate electrode thereof, which renders the device normally off.
Another method for obtaining a normally off state is to provide a P-type body over the gate region. Such an arrangement is shown in Saito et al., U.S. 7,038,252. Specifically, Saito et al. discloses a number of devices having insulated gates over the P-type body, which is disposed on the barrier layer of the active heterojunction of the device.
The presence of a PN junction in devices proposed by Saito et al. leads to gate leakage.
It is an object of the present invention to provide a normally off III-nitride semiconductor device which does not suffer from the drawbacks of the prior art.
A III-nitride power semiconductor device according to the present invention includes an N-polar III-nitride body having an N-polar face, a III-nitride buffer layer on the N-polar face, a III-nitride heterojunction that includes III-nitride barrier layer, and a III-nitride channel layer, the III-nitride heterojunction including a two-dimensional electron gas (2DEG), a first power electrode disposed over the heterojunction and coupled to the 2DEG, a second power electrode disposed over the heterojunction and coupled to the 2DEG, and a gate arrangement disposed over the heterojunction between the first and the second power electrode and including a P-type III-nitride body, and a gate contact, said P-type III-nitride body being disposed between the heterojunction and the gate contact.
According to one aspect of the present invention the barrier layer is disposed over the buffer layer and the channel layer is disposed over the barrier layer. Consequently, the PN junction is formed at the junction of the P-type III-nitride body and the channel layer resulting in confinement of electrons in the channel layer between the body and the barrier layers. This combination of doping and band gap alignments keeps the carriers from moving into the buffer layer during reverse bias conditions. By using an N-polar configuration, a barrier layer of larger band gap than the channel layer is achieved without the formation of a parasitic 2-dimensional electron gas at the interface between the barrier and the buffer layers.
In the preferred embodiment, the barrier layer is comprised of AlInGaN, and the channel layer is comprised of GaN. AlInGaN can be made with a larger band gap than the conventional material for the barrier layer, namely AlGaN, whereby leakage can be further reduced. That is, InAlGaN allows for a large barrier to electron injection, thereby preventing gate leakage. The P-type III-nitride body may be P-type GaN, or P-type AlInGaN. A device according to the preferred embodiment may further include a substrate, and a transition layer disposed between the substrate and the N-polar III-nitride body. The transition layer may be AlN or an oxide, while the substrate may made of silicon, SiC, sapphire, or a III-nitride material such as GaN.
A device according to the present invention is preferably a JFET. A JFET according to the present invention is more suitable for lower frequency applications (e.g. MHz range) because a PN junction under an insulated gate will result in accumulation and untimely and uncontrollable turning on/off of the device at low frequencies (e.g. MHz) leading to unacceptable circuit performance. In a device according to the preferred embodiment, the frequency dependent effects of an insulator above a p-type body is avoided.
A device according to the present invention may further include spacer field plates each disposed between the gate arrangement and a respective power electrode.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.